An apparatus of this type is disclosed in EP-A2-0 822 542. It has a four-quadrant photodetector, two diagonal sum generators, which sum signals from in each case two diagonally disposed quadrants of the photodetector, and a phase forming unit, which detects a phase difference between the diagonal summation signals. This phase difference is proportional to the deviation of a light beam, scanning the tracks of a recording medium, from the track centre. It is used as the track error signal in a tracking regulating circuit.
The known apparatus may be regarded as having the disadvantage that the phase angle of the diagonal summation signals relative to one another is determined incorrectly in certain cases. The result of this is that the track error signal assumes an incorrect value and, consequently, the tracking is not effected in an optimum manner. The phase angle is determined incorrectly for example when the scanning light beam deviates relatively far from the track centre and, in that case, signal influences occur which partly originate from the neighbouring track. In this case, it can happen that a correct phase angle cannot be determined.